<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        div{
            width: 0;
            height: 0;  border: 50px solid red; border-color: red transparent transparent transparent;
        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>